1. Field of the Invention
The present invention relates to a photopolymerizable composition and more particularly to a photopolymerizable composition comprising a polymerizable compound having ethylenically unsaturated bonds and a photopolymerization initiator of a novel composition and optionally a linear organic high molecular polymer, which is useful as, for instance, a light-sensitive layer, which is also sensitive to light from an argon ion laser source, of a presensitized plate for use in making a lithographic printing plate.
2. Description of the Prior Art
It has been well known in the art to form duplicates of images in accordance with a photographic manner employing a light-sensitive composition composed of a polymerizable compound having ethylenically unsaturated bonds and a photopolymerization initiator, which further contains a proper binder capable of forming a film and a heat polymerization inhibitor according to need. As disclosed in U.S. Pat. Nos. 2,927,022; 2,902,356 and 3,870,524, a light-sensitive layer composed of this kind of light-sensitive composition causes photopolymerization and is hardened when it is irradiated with light and thus, the areas exposed to light become insoluble in a developer. Therefore, a desired image can be obtained from such a light-sensitive composition by forming the light-sensitive composition into a proper film, irradiating the film with light through a negative transparency carrying a desired image and then removing only the non-exposed areas by selectively dissolving them with a proper solvent (hereunder simply referred to as "development"). Thus, it is a matter of course that this kind of light-sensitive composition is very useful for the manufacture of a presensitized plate for use in making a lithographic printing plate (hereunder referred to as "PS plate") and the like.
Moreover, it has also been proposed to incorporate, into such a light-sensitive composition, a photopolymerization initiator for the purpose of enhancing the sensitivity to light since if only the polymerizable compound having ethylenically unsaturated bonds is used, the resultant light-sensitive composition hardly exhibits a satisfactory sensitivity to light. Examples of such photopolymerization initiators which have been conventionally used include benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone and 2-ethylanthraquinone.
However, even if such a photopolymerization initiator is incorporated into this kind of light-sensitive composition, the resultant composition is inferior in the sensitivity to light (or the ability to cause quick hardening) and thus, it takes a long period of time required for irradiating it with light to form images. For that reason, in the case of the reproduction of very fine images, such images exhibiting a good performance cannot be obtained even if there is only a slight vibration in the imagewise exposure operations. In this case, it is further required to increase the dosage of radiant rays from a light source for exposure and, therefore, it is needed to use a means for diffusing a great deal of heat generated during the exposure operation. In addition, there is a strong tendency to cause the deformation of the film formed from the composition and the change in properties thereof because of such extreme heat generation.
Furthermore, these photopolymerization initiators exhibit a rather high ability to cause photopolymerization with respect to light sources emitting light having a wave length of not more than 400 nm (ultraviolet region) while the ability to cause photopolymerization with respect to those emitting light having a wave length of not less than 400 nm (visible light region) is substantially low. Therefore, the application of the photopolymerizable composition containing a conventional photopolymerization initiator is limited to only a very narrow range.
Under such circumstances, there has conventionally been some proposals concerning photopolymerization systems sensitive to visible light. For instance, U.S. Pat. No. 2,850,445 reports that certain kinds of photoreductable dyes such as Rose Bengale, Eosine and Erythrocin exhibit an effective sensitivity to visible light. In addition, as improved techniques thereof, there have been proposed a complex initiator system composed of a dye and an amine (Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") No. 44-20189); a system composed of a hexaaryl-bi-imidazole, a radical generator and a dye (J. P. KOKOKU No. 45-37377); a system of a hexaaryl-bi-imidazole and p-dialkylamino-benzylidene ketone (Japanese Patent Un-examined Published Application (hereinafter referred to as "J.P. KOKAI") Nos. 47-2528 and 54-155292); and a system composed of a substituted triazine and a merocyanine dye (J. P. KOKAI No. 54-151024). These systems proposed are certainly sensitive to visible light. However, the light-sensitive composition containing such a photopolymerization system is still insufficient in the photopolymerization velocity (or the sensitivity to light) and, therefore, it has been expected to develop a further improved photopolymerizable composition of this type.
Recently, it has been investigated to improve the light-sensitive composition in its sensitivity to ultraviolet rays and to develop a method for forming images utilizing laser beam and as a result, UV projection exposure methods, direct laser plate making methods, laser facsimile techniques, holography techniques and the like have already been put into practical use. Thus, there have been continued various studies to develop highly sensitive light-sensitive materials which are adapted to the application in such methods and techniques. However, the light-sensitive materials are still insufficient in the sensitivity.